We propose a factorial-design trial that simultaneously tests two interventions-one behavioral and one biomedical-to prevent HIV and STI (sexually transmitted infection) transmission in HSH ("hombres que tiene sexo con hombres"? Spanish for "men who have sex with men") in Peru. The behavioral intervention, entitled "Comunidades Positives," is based on a combination of social-action theory and diffusion of innovation, and is designed to reduce the frequency of unprotected sex with non-primary partners. The biomedical intervention will test the effectiveness of enhanced partner therapy for incident STIs, which involves providing presumptive therapy for certain bacterial and viral STIs to sexual partners. This intervention is designed to reduce the rate of re-infection and therefore, incident STIs. A total of 24 barrios with approximately 27 HIV-positive and/or STI-positive HSH in each barrio will be randomized to one of four cells, each cell corresponding to a combination of each of the two interventions, for a total of 648 subjects. The intervention will last 18 months and will be conducted in three coastal cities in Peru: Trujillo and Chiclayo in the north, and Lima, the capital. Outcomes will include incident STIs (HIV, HSV-2, syphilis, gonorrhea and Chlamydia) as well as unprotected intercourse. We will also model HIV infections averted based on sexual risk reduction and STIs treated and prevented. An expert, multidisciplinary team will conduct this trial, taking advantage of seven years of productive collaborative experience working on STI and HIV prevention in Peru. A substantial knowledge base, human resources, a collaborative network, and infrastructure are available for the trial. In addition, potential field sites and study participants are already involved in existing field trials.